The use of cell phones has become ubiquitous in today's society, with the use of such devices growing exponentially worldwide with each passing year. Consequently, planning of such networks has become more challenging as each base station, which receives and transmits cell-phone data, can process only a limited amount of such data on a particular range of frequencies or range of channels.
Several methods currently exist to address this increasing volume of data. A first method is to add a new network to the existing network by adding new radios to existing base stations, the new network operating at a different range of frequencies from the existing network. However this has the disadvantage of having to deploy the radios and associated equipment uniformly across the network, with each existing base station being outfitted with the new equipment to maintain consistent quality of service. The associated cost of this deployment is very high, and additionally over-utilizes precious spectrum resources. Non-uniform addition of radios leads to conditions where a cell-phone moving within the coverage area may have to switch networks as it moves from an area equipped with new radios to an area lacking new radios, a process known as a hard hand-off. Hard hand-offs are not reliable, leading to an increased number of dropped calls and thus degraded quality of service.
A second method of addressing increased volume is to add more base station sites in the network, operating at the existing range of frequencies. However it is important to carefully plan the location of these new sites as the cost of erecting a base station is high, including the equipment and cost of a long-term lease of a site. Introducing a new base station can be problematic in areas where several base station signals are detectable by a cell-phone located in these areas. Indeed signals from existing base station sites, even some distance from the location of a cell-phone, may also be detected by the cell-phone. The disadvantage of having more than three strong base station signals in an area is that interference between possible hand-off states will occur, again leading to an increased numbers of dropped calls and degraded quality of service. Current network planning methods do not adequately address this situation.
Hence, evaluating the location and characteristics of a proposed base station can be a challenging process as it is important to ensure both acceptable coverage of an area as well as limiting the number of possible hand-off states, a balance between too few available signals and too many. However, current techniques for optimizing base station locations are concerned mainly with the impact of land-cover and terrain on the coverage of an area, and ignore problems associated with accurately ensuring a limited number of hand-off states. If the base station, when built, improves network coverage in one area, but results in a detrimental number of hand-off states in areas which overlap with adjacent base stations, the result can be decreased quality of service or suboptimal capacity in these overlap areas and a need to either move the base station or build additional base stations to solve the problem, both unfavourably expensive options.
Currently, planning of base station locations is carried out in a two step method. In a first step, areas which are not being adequately covered by the existing network are identified. This is accomplished by measuring existing network signal strength along a grid in a given area. In spread spectrum systems such as CDMA networks, for example, a motorized vehicle equipped with a PN scanner and an antenna collects signal data from base stations located in an area thought to have poor coverage, or an increasing volume of cellphone calls. A PN scanner is a commercially available device which measures identifying signals, Ec, from individual base stations, known as pilot channels in a CDMA network, as well as total network signal activity or power spectrum density, Io, while concurrently recording the time and location of each measurement, location being measured using a Global Positioning System (GPS). Signal to noise ratio, Ec/Io, can than be generated for each measured channel and combined with the location data to obtain a map of existing network activity over a given area. Ec/Io is mapped instead of channel strength as the channel strength must be greater than the noise to be detectable by a cellphone. In this manner the degree of coverage of a network can be determined and areas which are receiving inadequate coverage or quality can be identified.
In a second separate step possible base stations sites are identified. This is currently carried out using one of a number of commercial software modeling programs, such as Marconi's PlanetEV. These software packages attempt to predict coverage of base stations in a network, including a proposed base station, by using existing digital terrain and land-cover maps of a given geographic area, usually taken from a geographic database, and subsequently modeling the signal strength of each channel at various points in the area using propagation models such as the Okumura-Hata model to predict the impact of land-cover on the signal, and the Longley-Rice model to predict the impact of terrain on the signal. Land-cover, defined as non-geographic features which affect signal strength, such as buildings or foliage, are important to these models as the location of these features have a major impact on the behaviour of a signal. Once the model predicts the best location of a new site, a lease may be negotiated with a landlord and the site is built. These software modeling programs may also be used to identify areas which are not being adequately covered. However a disadvantage of these methods is that the simulations are often inaccurate as the land-cover models they are based on assume unrealistic uniformity of buildings and foliage.
Another disadvantage to these methods is that they cannot directly take into account the effect of the new base station on the existing network except through the limited simulation models described above. Though some empirical techniques exist to improve the land-cover model, the simulation still has no accurate picture of existing network coverage. In any event, the result is that the program predicts the effect of a model base-station on a simulated network.
There remains therefore a need for an improved method of evaluating the proposed location of a proposed base station which more accurately takes into account the activity of the existing network.